Modulation of the immune system is desirable to treat a variety of diseases and disorders including, but not limited to, autoimmune diseases, infections, allergies, asthma, inflammatory conditions, spontaneous abortion, pregnancy, graft versus host disease, and cancers.
Antigen presenting cells (“APC;” e.g., CD1-expressing monocytes, macrophages, dendritic cells, and B cells) are required for protection against numerous diseases. APCs modulate immune responses by producing antibodies, cytokines, and growth factors; destroying infected and cancerous cells, and activating T cells.
T cells are lymphocytes that participate in multiple cell-mediated immune reactions, such as the recognition and destruction of infected or cancerous cells. Subsets of T cells, such as suppressor, cytotoxic, and helper T cells, mediate different immunologic functions. Suppressor T cells are responsible for turning the immune response off after an infection is cleared. Cytotoxic or “natural killer” T cells destroy infected or cancerous cells. Helper T cells produce cytokines that modulate the activity of cytotoxic T cells and/or antibody-producing B cells.
A subset of helper T cells, Th1 cells, secrete interleukin-1 (IL-1), IL-2, gamma interferon (INF-γ), and IL-2 which enhance cell-mediated responses such as cytotoxic T cell activity and inhibit both Th2 helper T cell activity and humoral immunity mediated by soluble antibodies. Due to their ability to kill antigen-presenting cells and their cytokine-mediated effector activity, Th1 cells are associated with vigorous delayed-type hypersensitivity reactions. Th2 cells, the other subset of helper T cells, are thought to inhibit cell-mediated responses and to enhance the humoral response. Th2 cells secrete IL-4, IL-5, IL-6, IL-9, IL-10, and IL-13 which activate B cell development and antibody production. T cells may also participate in immune deviation responses, such as the suppression of an ongoing immune response which may involve the secretion of TGF-β or IL-10 cytokines (Sonoda et al, J. Ex. Med. 190:1215-1255, 1999; Streilein et al., Hum. Immunol. 52:138-143, 1997; Hong et al, J. Ex. Med. 190, 1197-1200, 1999; Streilein et al., J. Immunol. 158:3557-3560, 1997).
To recognize a particular antigen bound to an antigen-presenting cell, most T cells express a highly specific T cell receptor (TCR) on their cell surface. The chains of the most common T cell receptors are called α and β. A second T cell receptor, found on a minor subpopulation of T cells, is composed of γ and δ chains. The genes for the α, β, γ, and δ chains of the T cell receptors have organizations similar to that of antibody genes: there are libraries of V, D, and J regions from which members are joined to form entire genes.
In contrast to most T cell subpopulations, which have diverse sequences for their TCR-a chain, invariant T cells have a highly conserved invariant TCR-α chain, Vα24-JαQ in humans and Vα14-Jα281 in mice, that pairs preferentially with human Vβ11 or murine Vβ8. These cells are either CD4+CD8− or CD4−CD8−. This invariant TCR is presumed to enable invariant T cells to recognize endogenous or pathogen-derived lipid antigens presented by nonpolymorphic MHC class I-like proteins, called CD1 family members. Humans have four CD1 proteins (CD1a, CD1b, CD1c, and CD1d), but mice have only a duplicated CD1d gene that is highly homologous to human CD1d. Human CD1d is expressed at high levels by thymocytes, at lower levels by B cells and monocytes, and by some cells outside of the lymphoid and myeloid lineages.
Many invariant T cells are distinguished by expression of several cell surface proteins otherwise found largely on natural killer (NK) cells, including CD161 (NKR-P1A) in humans, and a cell surface C-type lectin, NKR-P1C (NK1), in mice. This T cell subpopulation, referred to here as “invariant NK T cells,” represents a major fraction of the mature T cells in thymus, the major T cell subpopulation in murine liver, and up to 5% of splenic T cells in some mouse strains.
Murine and human invariant T cells also produce large amounts of the immunoregulatory cytokines IL-4 (a Th2 effector) and IFN-γ (a Th1 effector) in vivo in response to an anti-CD3 antibody or to CD1d. These cytokines allow the cells to participate in both Th2 and Th1 responses. The role of invariant T cells in augmenting the Th2 response, which appears to be protective in some autoimmune diseases, is further supported by the presence of defects in invariant T cells in a number of human and murine models of autoimmune diseases, including type 1 diabetes. Thus, alterations in the balance between Th1 and Th2 responses induced by invariant T cells may play a role in the development of autoimmune diseases.
Invariant T cells can also promote rapid Th1 immune responses and anti-tumor responses. Invariant T cells, which comprise a major fraction of the T cells in murine liver, can be stimulated by IL-12 to become active cytotoxic T cells and protect against liver metastases in tumor models. This conclusion was confirmed genetically through the generation of Jα281 knockout mice, which do not express the invariant Vα14-Jα281 TCR. These mice had markedly diminished numbers of invariant T cells and could not mediate IL-12 induced tumor rejection. Other studies showed that IL-12 administration no longer induced an early IFN-γ response in the spleen and liver of CD1d knockout mice, which are invariant T cell deficient. In addition, data from human patients shows fewer invariant NK T cells and reduced Th1-like responses in patients with advanced cancer. The anti-tumor response of activated invariant T cells could be partially mediated by their CD1d specific cytotoxicity and NK/LAK cell-like toxicity. Other regulatory functions of invariant T cells, possibly through cytokine production or interactions with antigen presenting cells (APCs), may also play important roles in anti-tumor immune responses.
Invariant T cells may also have a role in the pathogenesis of spontaneous abortion. Stimulation of decidual invariant T cells in mice by administration of a ligand for invariant T cells provoked abortion in pregnant mice. The perforin-dependent killing and production of IFN-γ and tumor necrosis factor-α by the invariant T cells were required for this induction of abortion.
In contrast to human peripheral blood in which invariant T cells are the major CD1d-reactive subpopulation, human and mouse bone marrow and human liver have T cell populations dominated by CD1d-reactive noninvariant T cells using diverse TCRs which can also produce a large amount of IL-4 and IFN-γ. These CD1d-reactive noninvariant T cells can be either NK or non-NK T cells, and they function similarly to CD1d-reactive invariant T cells. The CD1d-reactive noninvariant T cells in bone marrow may have a role in suppressing graft versus host disease, and both populations may enhance graft versus leukemia responses. In the liver, these T cells may protect against infections, such as Hepatitis C infections, but may also cause damage due to their Th1 response. Additionally, we found that CD1d-reactive NK T cells are critical for immune tolerance to antigens in the anterior chamber of the eye, an immune privileged site (Sonoda et al., supra). Such mechanisms may also be important in the maintenance of peripheral tolerance.
Parasitic glycosyl-phosphatidylinositols derived from Plasmodium, Trypanosoma, or Leishmania have been recently shown to stimulate murine CD1d-reactive invariant Vα14 NK T cells. In addition, an α-galactosylceramide (α-GalCer) lipid, which was isolated from marine sponge in a screen for anti-tumor activity, is a CD1d-presented antigen. α-GalCer is an example of an agent which can be used to expand human CD1d-reactive invariant T cells from umbilical cord or peripheral blood samples that are first enriched for invariant T cells by purification using an anti-Vα24 antibody. The enriched Vα24+ cells are cocultured in the presence of α-GalCer and purified antigen-presenting cells (APCs). However, it would be desirable in a clinical setting to use a simpler method for enhancing protective immune responses that does not require the modulation of CD1-reactive T cells.
Thus, there exists a need to specifically modulate the immune system for the prevention and treatment of diseases and disorders such as diabetes, autoimmune diseases, infections, allergies, asthma, inflammatory conditions, spontaneous abortion, pregnancy, graft versus host disease, and cancers.